The present invention relates to a power control circuit of induction heating system, and more particularly to a dual push-pull induction heating drive circuit.
The induction heating principle has been widely employed for general heating purposes. The case in point is an induction cooker, which is selected as an example to illustrate ingenuity and creativity disclosed in the present invention. Therefore, the present invention is not restrictive to an induction cooker.
An induction cooker is a cooking apparatus employing the principle of electromagnetic induction. In other words, the bottom of a cookerware is caused to bring about heat by eddy current induced by an externally applied alternating magnetic field. Such induction cooker has been well received by consumers at large in view of the facts that it does not generate lethal gas of carbon monoxide and that it is easy and safe to use and further that it is relatively heat efficient.
The control principle of an induction cooker of prior art is illustrated in FIG. 1. A pulse signal having a predetermined width is generated by means of an oscillation wavewidth modulator 1 (OSC/PWM) and is subsequently amplified by an amplifying circuit 2 (AMP). The amplified pulse signal is then used to drive a power transistor Q1 to generate continuously a switching action of ON/OFF so as to condition the output induction coil L1 to generate an alternating magnetic field of high frequency, which induces the induction cooker to bring about heat. The induction element L2 and the capacitor C2 are used for filtering. As shown in FIG. 2 illustrating the dynamic voltage current wave form of the power transistor Q1, the time T1 that is required to drive the power transistor Q1 is about 5-25 us. Therefore, the output power of the induction cooker can be changed by adjusting the time T1. In other words, a greater output power is achieved by prolonging the time T1. The time T2 is dependent on the natural resonance frequency formed by the output coil L1 and the capacitor C1 and is on the order of 15us. With the power voltage of 110 V, such induction cooker can generate an output power ranging between 200 W and 1200 W.
If the power supply voltage is increased to 220 V, the output coil L1 of an induction cooker intended to operate on power supply voltage of 110 V must be doubled accordingly. In addition, the capacitance of the capacitor C1 must be reduced by one half so as to permit the time T2 of the power transistor Q1 to remain unchanged. With such adjustments being made, the induction cooker should be able to generate the output power ranging between 220 W and 1200 W. However, such modification is by no means desirable. As far as an induction cooker intended to operate on the power voltage of 110 V is concerned, the maximum collector voltage Vc(max) and collector current Ic of the power transistor Q1 are 650 V and 45 A respectively. As a result, the electric specifications for selecting the power transistor are voltage tolerance Vcbo of 900 V and current tolerance Ic of 60 A. The maximum collector voltage Vc(max) of the power transistor Q1 of an induction cooker operating on the power voltage of 220 V can be doubled if the number of coil of output coil L1 is also doubled in conjunction with a one half reduction in capacitance of the capacitor C1. Therefore, only the power transistor having the voltage tolerance Vcbo of at least 1400V can be used for such modification as described above. The cost of making such modified induction cooker is relatively expensive in view of the fact that the power transistor with Vcbo of 1400 V is rather expensive and difficult to come by.
In order to reduce the production cost of the induction cooker operating on power voltage of 220 V as described above, the number of coil of output coil L1 may be appropriately reduced while the capacitance of the capacitor C1 is increased, thereby resulting in a substantial reduction in the maximum collector voltage Vc(max) of the power transistor Q1. As a result, a power transistor having a lower voltage tolerance Vcbo and having a lower price tag can be selected for use. However, such practice is not desirable, because the power transistor Q1 is subjected to a possible damage caused by the current overload, in view of the facts that the transient current Ict of the power transistor Q1 tends to increase in the wake of alterations made in the output coil L1 and the capacitor C1, and that the time T2 required to drive the power transistor Q1 is shortened to result in a reduction in output power.
It is apparent that an induction cooker of prior art is provided with only one power transistor or single push-pull induction heating drive circuit. The output power of an induction cooker can be increased by means of switching circuits of dual push-pull type, as shown in FIG. 3, in which two power transistors Q1 and Q2 are seen communicating each other. When power transistor Q1 is driven, the current I1 is permitted to pass the power transistor Q1, the induction coil L1 and the capacitor C3. As soon as the power transistor Q2 is driven the current I2 passes through the power transistor Q2, the induction coil L1 and the capacitor C4. As a result, the induction coil L1 generates an alternating magnetic field. The switching circuits of dual push-pull type have been employed in the switching power supply. The load side of the dual push-pull circuit of the switching power supply is rectifying filtering power source of constant frequency. It is therefore suggested that the deed of regulating the control circuits driving the power transistors Q1 and Q2 is a simple matter and that a specialized integrated circuit is available for such application. It is technically difficult to apply the switching circuits of dual push-pull type to an induction cooker in view of the facts that the load side of the induction coil L1 of the dual push-pull circuit is the body of cookware in contrast to the switching power supply in which the load side is a rectifying filtering power source, and that the output power of the induction cooker depends on the modulation of the drive pulse width of dual push-pull switching circuit, which is accomplished by means of a control circuit. It is therefore apparent that an induction cooker can not be suitably provided with control circuits serving to drive alternately the power transistors Q1 and Q2 of dual push-pull circuit of the switching power supply.
The induction cooker of prior art is provided with single push-pull circuit and with a complicated power controlling circuit. On the contrary, the present invention is provided with switching circuit of dual push-pull type to control the mechanism of induction heating with a view to constructing the induction circuit of relatively small size and of high efficiency at a low cost for commercial application.